Scratch round opening of Porsche's exhaust (7)
I believe the answer is:
deplete
'exhaust' is the definition.
(I know that exhaust is a kind of deplete)
'scratch round opening of porsche's' is the wordplay.
'scratch' becomes 'delete' (I've seen this in another clue).
'round' is an insertion indicator.
'opening of' says to take the initial letters.
The initial letter of 'porsches' is 'p'.
'delete' enclosing 'p' is 'DEPLETE'.
(Other definitions for deplete that I've seen before include "Drain" , "Exhaust (stocks)" , "Use up, exhaust" , "Empty, reduce, exhaust" , "bankrupt" .)